312 J. A. POLLOCK, A. B. B. RANCLAUD AND E. P. NORMAN. 
change sign in the near neighbourhood of the heated surface. 
In this case the potential difference between the electrodes 
for zero current, with appropriate sign, must be less than 
the value of the surface discontinuity of potential which 
would be due to the projection, alone, of that class of ions 
which conditions the sign of the potential difference. 
With the apparatus described in a previous paper’ the 
potential difference for zero current has been found in the 
case of glowing carbon at various temperatures. The 
experiments were a continuation of the work already 
described, and in the paper just mentioned will be found 
full details of the method of investigation. The observa- 
tions, which fulfil the condition that the values should be 
independent of the distance separating the electrodes, are 
shown in figure 1. For zero current the hot carbon was 
when dystance \between| carbons = 2mm, 
- = 
E 
| 
i al 
Rw 
ie 
| | 
| 
Ee 
ES as 
im for 
| 
; 
9 a 
Volts a C. 
« 
a 
CCC EET oe 
e 
1300 1500 1700 1900 2100 2300 2500 2700 2900 3i00 3300 3500 3700 
Temperature Absolute. 
Potential differences for zero current. 
+ Pollock and Ranclaud, this Journal p. 201. 
